


golden rule of fūinjutsu

by tripletmoons



Category: Naruto
Genre: Kagami is a disaster but we love him, M/M, time-travel
Language: English
Status: Completed
Published: 2018-11-14
Updated: 2018-11-14
Packaged: 2019-08-23 17:19:04
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 1,356
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16623149
Author URL: https://archiveofourown.org/users/tripletmoons/pseuds/tripletmoons
Summary: On the first day of lessons, back when Tobirama was Tobirama-sensei and before Kagami ever put ink to paper, Tobi handed down the golden rule of sealing. The most basic - most general - rule that every practitioner must follow: never activate a Uzushio seal without fully understanding it.





	golden rule of fūinjutsu

Tobirama has a lot of rules when it comes to fūinjutsu. To be fair, most of these rules are specific rather than general. _Don’t test explosive seals when sleep deprived, Kagami. Don’t try to incorporate slang into sealing language, Kagami. Never talk about sexual-stimulation seals with my sister-in-law again, Kagami._ But on the first day of lessons, back when Tobirama was Tobirama-sensei and before Kagami ever put ink to paper, Tobi handed down the golden rule of sealing. The most basic - most general - rule that every practitioner must follow: never activate a Uzushio seal without fully understanding it.

Kagami is reminded of this golden rule when he finds himself in front of a much, much younger version of his beloved husband. A version of his husband with slightly-chubby cheeks and an _oncoming sword strike holy SHIT_.

Kagami makes a _very manful_ noise of shocked, surprise; reflexively parrying the descending blade with his tanto. Impressively composed - only a slight wideness of the eyes and slackness to the limbs betraying surprise - his future-husband disengages. Which is good because Kagami is not dealing with his shock nearly as well and couldn’t have blocked a Genin’s punch at this point, much less a hit from teenage Senju Tobirama. Even limned with the trappings of youth, Tobi has intimidatingly thick biceps.

“Oh, thank the gods. That was close.” Someone huffs from behind him. The voice is vaguely familiar, ominously familiar; straight out from some of his oldest memories. Kagami’s joints lock.

Stiffly, he turns to stare at a living, breathing, _oh-shit-i-just-saved-his-life_ Uchiha Izuna. Tobirama’s rants regarding the sanctity of the space-time continuum clamor discordantly around his skull. Feeling a little faint, he looks around, taking in the battlefield filled with Senju and Uchiha _fighting each other._

“Oh,” Kagami moans, “nooooo…”

Out of the corner of his eye, Uchiha _alive_ Izuna’s expression twists with offence and sudden suspicion. “Wait, who the hell are you? An Uchiha, obviously, but I don’t recognize-.” But before he can finish, a voice booms across the clearing, so thick with killing intent the fighting ninja freeze like hares before a hawk.

" _Kagami… What have you_ done _?!_ ”

Kagami turns, again, to see his husband. Like, the man he married in the flesh, not the younger version, holding up a Kumo shinobi by her broken neck and scowling thunderously. Truly, a vision of death.

“Uh-.”

Tobirama’s sweeps his eyes cuttingly across the battlefield. If possible, it becomes even more silent, even more still. The sound the dead Kumo shinobi makes when she hits the ground reverberates. Tobirama brings down his extended arm, a grimace flickering at the corners of his mouth. _Wait a minute._ Kagami recognises that barely-there grimace; it’s a pain-grimace, not a disgust-grimace.

He recalls why he so desperately pushed his chakra into the Uzushio seal in the first place.

In a blink, Kagami shunshin’s to his husband’s side, weaseling his hands up and under armor to press against the wound on his chest. He can’t help the distressed noise that escapes when he feels the cut, deep and positively _weeping_ blood.

Tobirama stays tense as a board under Kagami’s hands, but his voice is forcibly calm when he speaks. “It is not that bad.”

“You’ve been skewered!”

Tobirama huffs, sliding a glowing green hand up aside his. Kagami can feel the skin begin to knit together but doesn’t stop holding on. The armor-piercing strike came perilously close to hitting the left lung. _Perilously close._ That lung has been compromised since the Kinkaku incident five years ago. One hit: instant death.

“There, I’m fine. Now: what-.”

“Oh my gods.”  He breaths.

“Kagami.”

“I was almost widowed.”

“ _Kagami_.”

“Amaterasu, that was close. Too close. Oh lords above, I’m too young and good-looking to be made a window. The Elders wouldn’t let me mourn. I’d be remarried within the month and and miserable and-.”

“ _Kagami_.” Tobirama smooshes a hand against his face. “What,” he hisses, “is the first rule of fūinjutsu.”

Kagami again recalls the golden rule, recalls his husband theorizing; “Paradoxes change the past and doom the future.”

Carefully, he smooths his hands over Tobirama’s bloody shirt, pulls a safe distance away, and smiles, projecting all the love and affection he has in his body. He's an Uchiha, so there's a lot. “Tobi, dear-heart, light of my life.”

His husband bristles, aura of death mushrooming out.

“Listen, I _uh_ was very panicked, okay. I saw the protective elements of the seal and assumed, well, assumed it _wasn’t a time travel seal_ because I am a reasonable person and reasonable people don’t...”

“The golden rule, Kagami. It’s a very simple rule.”

Kagami changes tactics. “I am the father of your child and I love you very much so I-.”

"I love you too." Tobirama doesn’t soften. “We have no children.”

“No, but we _could_ and really that’s what matters here so-.”

“What. The. _Fuck_ . WHAT IS GOING ON HERE!?” Someone shouts. Kagami is thankful to have his desperate babble cut off until the familiarity of the voice - and the _tone -_ registers.

“Madara-sama!” Kagami eeps _,_ ducking behind his husband on instinct, visions of being thrown into a Koi pond dancing through his head.

“What is this? What is-? _What_?”

As he is thoroughly dwarfed by Tobirama - _damn Senju blood_ \- Kagami has to peer around his husband in order to see. _Damn Uchiha curiosity!_

The young versions of Madara, Izuna, Hashirama, and his husband are standing together and staring. Madara has dissolved into red-faced sputtering. Behind them, the rest of the battlefield watches on like they’re viewing a theatre production. Two clans united in enjoying drama.

Hashirama collects himself first and asks, tentatively, “Tobi?”

“Brother.” Tobirama grunts.

Kagami half expected him to deny it, but then again: you can’t really mistake the striking albino hunk of muscle that is Senju Tobirama, no matter the age.

Hashirama _glows._

“Tobi, look at you, you get so _big.”_ He looks ready to rush over for a hug, but young-Tobirama grabs him by the sleeve without breaking the stare-off with his older self.

“Kagami.” Izuna says, voice heavy with realization, with betrayal. “Uchiha Kagami. Kagami- _chan.”_

Madara damn near breaks his neck with how fast his gaze darts down to Kagami, whose retreat behind Tobirama’s back is stopped by Tobirama himself.

“Kagami.” Madara gasps, betrayal writ all over him.

Kagami is plucked up and propped down right in full view, unable to escape his young Clan Head’s accusing gaze. Tobirama’s hands are like iron bands against his shoulders, holding him in place.

“Traitor.” Kagami whispers.

Tobirama’s silence heavily implies that this is Kagami’s punishment for the sin of breaking the first rule of fūinjutsu. The silence also implies that if Tobirama had more chakra to spare he'd create a pond for his Madara to throw Kagami into, right here and now. 

“Hi, Madara-sama.” Kagami waves sheepishly. “May I just say, your older self looks so like you, it’s like he hasn’t aged a day...”

Madara squints. “Kagami,” he says slowly, “why are you with Senju Tobirama.”

“Ah, well you see…” Kagami can’t make himself finish the sentence. He will never forget Madara-sama’s response to being told they were marrying. He does not want a repeat.

Tobirama, of course, has no such desire. He _thoroughly_ enjoyed Madara-sama’s response to being told they were marrying. “Kagami is my husband.” 

Madara takes that like a hit to the face. Red crawls up his chest, flooding his face; the tendons of his neck strain tellingly. Izuna reacts similarly.

Typically, Kagami would be reacting to all that rage, but he’s too busy staring at young Tobirama, who lacks the iron-cast control older Tobirama possesses. Younger Tobirama, who wears the same expression Kagami’s Tobirama wore when they first kissed. The stunned once-over he gives Kagami is more than a little flattering. As is the intensifying blush.

_So cute!_ He feels himself go weak at the knees. The hands on his shoulders are now effectively holding him up. “Worth it.” He says. “ _Totally_ worth it.”

Tobirama - the man he loves with all of his heart, his husband - releases his shoulders, allowing Kagami to drop to the ground like a sack of rocks. 

**Author's Note:**

> This isn't super edited - and could for sure be better - but I hope you like it anyway. It spent a couple of days rattling around my head before I just had to let it out.


End file.
